1. Field of the Invention
The present invention relates to a composite ceramic structure, and more particularly to a high strength composite ceramic structure suitable for structural members for use in engines and turbines and a process for producing the same. Moreover, the present invention relates to a sintered ceramic product having a surface configuration accurate in dimension as being an accurate replica of the surface contours of a mold.
2. Description of Related Art
There have been generally known SiC and Si.sub.3 N.sub.4 which have an excellent heat resistance as engineering ceramics suitable for structural members in engines, turbines and the like. Since the SiC and Si.sub.3 N.sub.4 are compounds having strong covalent bonds, however, these nitrides alone are difficult to sinter and hence require sintering aids to produce sintered shapes.
For example, it has been recognized that when Si.sub.3 N.sub.4 is sintered under ambient pressure, an addition of Y.sub.2 O.sub.3 or Al.sub.2 O.sub.3 permits the production of highly dense sintered shapes. However, because the glass phase produced with the sintering aids is softened at high temperatures, the mechanical properties of the sintered shapes are lowered at high temperatures. In order to prevent the reduction in strength at high temperatures, an attempt has been made to use as small amount of the sintering aids as possible, or to crystallize the glass phases at the grain boundaries attributable to the sintering aids. However, no complete solution has been achieved yet.
On the other hand, though an enhancement in the strength of the composites at high temperatures by incorporating whiskers of SiC, Si.sub.3 N.sub.4, and the like, so-called a reinforcing technique with fibers has been proposed [see, J. Ceram. Soc. of Japan, 91, [11], (1983) pp.491-497], this also employs the sintering aids causing a significant reduction in the mechanical properties of the sintered shapes.
Another technique for sintering the ceramic composites comprising two kinds of inorganic compounds different in characteristics from each other without using any sintering aids, for example, a bonding reaction process has been known (see, Japanese Patent KOKAI (Laid-open) No. Sho 61-101465). Since this prior process employs metallic silicon particles of an average size of 0.5 .mu.m or more, however, the resulting materials studied were macroporous sintered shapes having a maximum pore size of not less than 30 .mu.m in which relatively large silicon nitrides bond spaces of SiC particles. For this reason, attainment of high strength of the sintered shapes has been limited. There has been studied no method for supplying silicon nitrides from a gaseous source. With respect to ceramic composites comprising two or more kinds of inorganic compounds, requirements for achieving high bonding strength have not been clarified and no high strength ceramic has been obtained so far.
SiC and Si.sub.3 N.sub.4 which have an excellent heat resistance are known as engineering ceramics generally suitable for structural members for use in engines, turbines and the like. There are techniques for sintering SiC and Si.sub.3 N.sub.4 such as an ordinary pressure method, a high pressure method, and a reaction-sintering method. Among them the ordinary pressure method and the high pressure method suffer from deformation and less accuracy in dimensions as causing a rate of variation in size of 15 to 20% so that a high level of technique is required. On the other hand, the reaction-sintering method is known to cause a lower rate of variation in size at the sintering process than those by other sintering methods, but still in the range of 1 to 1.5% [Japanese Patent KOKAI (Laid-open) No. Sho 58-140375]. Materials expected to have a lower rate of variation in size at the time of sintering include Si.sub.3 N.sub.4 binder which has been used heretofore in refractories, but which has only a mechanical strength as low as 50 MN/m.sup. 2 [see, Japanese Patent KOKAI (Laid-open) No. Sho 58-88169], unsuitable for use in structural members. The high rate of variation in size at the time of sintering imposes time consumption and high cost on processing after sintering, which is one of the reasons why the engineering ceramics have not been widely used. Therefore, there has been heretofore no technique, nor practical method of producing ceramics excellent in the accuracy in dimension.